One of the difficulties in making containers on injection blow molding machines is that there are limitations on the length of the container with respect to its diameter or cross section. One way to increase the length-to-diameter ratio is to use longer core rods but this has the disadvantage that the long core rods do not have the stability of shorter ones and increasing the cross section of the core rod makes it unusable on conventional machines.
This invention uses a different expedient for increasing the length-to-diameter ratio without changing the core rods. A novel blowing mold is used which causes the parison to be blown first in a lengthwise direction and then to the full cross section of the blow mold cavity. This differential blosing action is obtained by having a tube which fits into the blow mold cavity from the end opposite that through which the core rod projects, and with the tube in axial alignment with the core rod and slightly larger than the cross section of the parison.
The tube prevents radial expansion of the parison but permits it to elongate when the blowing operation starts. As the blowing operation continues, the tube is withdrawn from the blowing mold and the parison expands progressively starting from the neck end of the core rod.
The invention requires a different blowing mold, but different molds are required for every different container anyway. The novel mold of this invention has simple apparatus for withdrawing the tube from the mold and it can be added to conventional blow molding machines.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.